The importance of roads and highways has been appreciated since the time of the Roman Empire. Stone blocks, wood blocks, vitrified brick and natural asphalt (bitumen) has been used to pave roads and highways for centuries. However, at the beginning of the automobile era, must rural roadway surfacing consisted of broken stone or gravel. Such roads were often rough, dusty and clearly inadequate for modern automobile and truck traffic.
Today roads, highways, driveways and parking lots are often paved with asphalt concrete. Pavement can be made with asphalt concretes which are dust-free, smooth, and which offer the strength required for modern automobile and heavy truck traffic. Asphalt concrete is generally made by mixing aggregate (sand and gravel or crushed stone) with the proper quantity of an asphalt cement at an elevated temperature. The hot asphalt concrete is then placed by a layering machine or paver on the surface being paved and thoroughly rolled before the asphalt concrete mixture cools. The asphalt concrete is normally applied at a thickness varying from about 25 to about 100 millimeters.
Asphalt concrete pavements can be made to be very smooth which offers outstanding frictional resistance for vehicles operating thereon. Such asphalt concrete pavement can also be repaired simply by adding additional hot asphalt concrete to holes and other types of defects which develop in the surface. Asphalt concrete pavements can also be upgraded easily by adding additional layers of hot asphalt concrete to old surfaces which are in need of repair.
Even though asphalt concrete offers numerous benefits as a paving material, its use is not trouble free. One major problem encountered with asphalt concrete pavements is the loss of the adhesive bond between the aggregate surface and the asphalt cement. This breaking of the adhesive bond between the asphalt cement and the aggregate surface is known as "stripping". The stripping of asphalt binder from aggregate surfaces results in shorter pavement life and many millions of dollars of maintenance work on American highways each year. Reduction of this stripping tendency is of great interest when trying to improve conditions of roads and lowering these maintenance costs.
Stripping is a very complex problem which is dependent upon numerous variables, such as asphalt cement characteristics, aggregate characteristics, weather conditions, traffic conditions, construction practices used, and the use of anti-strip additives. Five different mechanisms by which stripping of asphalt cement from an aggregate surface may occur have been identified by A. R. Tarrer and Vinay Wagh in a Literature Review prepared for the Strategic Highway Research Program, which is titled, "The Effect of the Physical and Chemical Characteristics of the Aggregate on Bonding". These five mechanisms include detachment, spontaneous emulsification, displacement, pore pressure, and hydraulic scoring.
Over the years, various methods have been developed to reduce stripping tendencies. For instance, amines and polyamines are frequently applied to the surface of the aggregate prior to mixing it with the asphalt cement in making asphalt concrete. Amines and polyamines are known to act as anti-stripping agents in such applications. However, the use of amines and polyamines does not totally solve the problem of stripping and their use presents environmental problems. Lime (calcium hydroxide) is also conventionally used to treat aggregate to reduce stripping tendencies. However, the use of lime in such applications does not totally solve the problem of stripping. Additionally, lime is a corrosive material which causes equipment maintenance problems. Lime has also been known to burn the skin of workers using it in such applications.